Generally, coating is applied to the inner surfaces of food and beverage cans in order to prevent the corrosion of the can material due to the wide variety of contents therein. The coating composition applied to the inner surfaces of food and beverage cans is required to have the following properties. First, the coating composition should be free from toxicity, and have sufficient curability (curability); second, the coating composition is required to have excellent processability during molding (processability), tolerance during heat sterilization (retort resistance), no blister or blushing when contents containing salt or that exhibit acidity are subjected to heat sterilization (content resistance), impact resistance after heat sterilization (dent resistance), and the like. In recent years, the use of bisphenol-type epoxy resins and the like has been avoided since there is a concern that such resins function as extrinsic factor endocrine disrupters (hereinafter, these may be referred to as environmental hormones).
Currently, polyvinyl chloride-based resins and epoxy-phenol-based resins are often used as a base resin for coating compositions for the inner surfaces of cans. However, because these resins have the serious problems indicated below, there is a demand for the development of a coating agent for inner surfaces that will replace the above-mentioned resins.
Although polyvinyl chloride-based resins exhibit excellent retort resistance, content resistance, and processability, the vinyl chloride monomer remaining in the resins are indicated to have serious health dangers such as carcinogenicity. Furthermore, when the cans are incinerated, there is a risk that highly toxic and corrosive chlorine gas, hydrogen chloride gas, and extremely toxic dioxin may be generated from the polyvinyl chloride-based resins, which may cause the corrosion of the incinerator and environmental pollution. In addition, polyvinyl chloride-based resins have insufficient adhesion with metals, which are materials for cans; therefore, it is necessary to perform a surface treatment using an epoxy resin before coating, complicating the coating process.
Epoxy-phenol-based resins require a high baking temperature; accordingly, poor appearance, such as foaming, may easily occur during baking. Furthermore, as described above, bisphenol A contained in epoxy resins may function as an environmental hormone.
In addition to the above-described processability and retort resistance that are required for coating compositions for inner can surfaces, coating compositions for the external can surfaces are also required to have countermeasures against environmental hormones.
In order to solve the above problems, the application of a polyester-based resin to a coating composition for the inner surfaces of cans has been attempted, therein the polyester-based resin does not generate a toxic gas and/or corrosive gas during incineration, and does not contain an environmental hormone, such as bisphenol A, in the coating film. However, a coating composition resin and a coating composition resin composition that simultaneously exhibit satisfactory processability, curability, retort resistance, content resistance, and dent resistance that are suitable for use in cans have not been obtained.
Patent Literature 1 discloses a polyester resin that has excellent flavor preservability, that is environmentally friendly and highly safe from a health standpoint, wherein the polyester resin is used for a coating composition for a metal package. The polyester resin is obtainable by copolymerizing a dicarboxylic acid component composed of 80 to 100 mol % of terephthalic acid and 0 to 20 mol % of isophthalic acid with a glycol component composed of 60 to 90 mol % of propylene glycol, and 10 to 30 mol % of ethylene glycol or 1,3-butylene glycol. However, such a technique has drawbacks in regards to achieving sufficient processability and/or curability.
Patent Literature 2 discloses a polyester resin for use in a can coating composition, the polyester resin being obtainable by reacting an acid component composed of 80 to 100 mol % aromatic dicarboxylic acid containing 70 to 95 mol % terephthalic acid and 0 to 20 mol % polybasic acid other than aromatic dicarboxylic acid with a glycol component comprising 2-methyl-1,3 propanediol and 1,4-cyclohexane dimethanol as essential components, wherein among the glycol components constituting the polyester resin, the content of 2-methyl-1,3 propanediol is 25 to 50 mol % relative to the total glycol components, and wherein the total weight of terephthalic acid and 1,4-cyclohexane dimethanol falls within the range of 45 to 65 wt % relative to the polyester resin. Patent Literature 2 also discloses that the resulting polyester resin is excellent in processability and stain resistance. However, the above technique has drawbacks in regards to obtaining satisfactory processability and/or curability.
Patent Literature 3 discloses a coating composition comprising hydroxy-containing polyester resin and phenol-formaldehyde resin as essential components, and further comprising benzoguanamine-formaldehyde resin and/or blocked polyisocyanate, etc. Patent Literature 3 also discloses that the resulting coating composition is excellent in processability, curability, and retort resistance. However, the above technique has drawbacks in regards to obtaining satisfactory processability and/or curability.